The problem with any MANUAL valve arrangement is the chance, Low that it may be, of leaving both valves open with no external tank.
The Extend-A-Stay and the Stay-A-While both use automatic check valves to prevent just that from happeing.
This is why home made systems frankly scare me. I don't wish to be parked within 100 yards of a home made system. But I've no problem with the Extend-A-Stay. Use one myself.
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